Excerpts from the link:
Why she sank - did they see an iceberg or not?
On the night of April 14, 1912, though, only a few days into the Titanic's maiden voyage, its Achilles' heel was exposed. The ship wasn't nimble enough to avoid an iceberg that lookouts spotted (the only way to detect icebergs at the time) at the last minute in the darkness. As the ice bumped along its starboard side, it punched holes in the ship's steel plates, flooding six compartments. In a little over two hours, the Titanic filled with water and sank.
And as for her sister ships:
Harland & Wolff, now an engineering and design firm, flatly rejects the notion that its rivets were weak. Tom McCluskie, the company's retired archivist, points out that Olympic, Titanic's sister ship, was riveted with the same iron and served without incident for 25 years, surviving several major collisions, including being rammed by a British cruiser. "Olympic deliberately rammed a German submarine during the First World War and cut it in half," says McCluskie. "She was plenty strong." The Britannic sank after hitting a mine during World War I. Both ships were strengthened after the Titanic disaster with double hulls and taller bulkheads, but their rivets were never changed.
Regarding the conspiracy theories:
Some conspiracy theorists believe that the company's silence was a sign of a coverup, and that the post-disaster retrofitting of Titanic's sister ships proves Harland & Wolff knew its ship was flawed. But most historians come to a different conclusion. "The fact that the ship broke up on the surface does not mean she was weak," says Long. When 38,000 tons of water filled its bow, pushing the stern up even 11 degrees out of the water, the ship was loaded beyond its capacity and cracked in two.
Could the Titanic have been stronger? Certainly. Higher-quality rivets or a thicker hull might have kept the ship afloat longer. But ultimately, the Titanic was designed to be a passenger liner, not a battleship. "[The ship] was built to the best of their knowledge at the time and to the proper standards. Nothing could have survived what happened to it," says McCluskie. Extensive forensic analysis of the wreckage has, in a way, brought the story of the Titanic to a familiar place. "The ship," says Foecke, "was just not designed to run into icebergs." When it did, nothing could stop its journey to the bottom.